Triphosphorus pentanitride

Triphosphorus pentanitride is an inorganic compound with the chemical formula P3N5. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. While it has been investigated for various applications this has not led to any significant industrial uses. It is a white solid, although samples often appear colored owing to impurities.

Synthesis
Triphosphorus pentanitride can be produced by reactions between various phosphorus(V) and nitrogen anions (such as ammonia and sodium azide):


 * 3 PCl5 + 5 NH3 → P3N5 + 15 HCl
 * 3 PCl5 + 15 NaN3 → P3N5 + 15 NaCl + 20 N2

The reaction of the elements is claimed to produce a related material. Similar methods are used to prepared boron nitride (BN) and silicon nitride (Si3N4); however the products are generally impure and amorphous.

Crystalline samples have been produced by the reaction of ammonium chloride and hexachlorocyclotriphosphazene or phosphorus pentachloride.


 * (NPCl2)3 + 2 [NH4]Cl → P3N5 + 8 HCl
 * 3 PCl5 + 5 [NH4]Cl → P3N5 + 20 HCl

P3N5 has also been prepared at room temperature, by a reaction between phosphorus trichloride and sodium amide.


 * 3 PCl3 + 5 NaNH2 → P3N5 + 5 NaCl + 4 HCl + 3 H2

Reactions
P3N5 is thermally less stable than either BN or Si3N4, with decomposition to the elements occurring at temperatures above 850 °C:


 * P3N5 → 3 PN + N2
 * 4 PN → P4 + 2 N2

It is resistant to weak acids and bases, and insoluble in water at room temperature, however it hydrolyzes upon heating to form the ammonium phosphate salts [NH4]2HPO4 and [NH4]H2PO4.

Triphosphorus pentanitride reacts with lithium nitride and calcium nitride to form the corresponding salts of PN4(7−) and PN3(4−). Heterogenous ammonolyses of triphosphorus pentanitride gives imides such as HPN2 and HP4N7. It has been suggested that these compounds may have applications as solid electrolytes and pigments.

Structure and properties
Several polymorphs are known for triphosphorus pentanitride. The alpha‑form of triphosphorus pentanitride (α‑P3N5) is encountered at atmospheric pressure and exists at pressures up to 11 GPa, at which point it converts to the gamma‑variety (γ‑P3N5) of the compound. Upon heating γ‑P3N5 to temperatures above 2000 K at pressures between 67 and 70 GPa, it transforms into δ-P3N5. The release of pressure on the δ-P3N5 polymorph does not revert it back into γ‑P3N5 or α‑P3N5. Instead, at pressures below 7 GPa, δ-P3N5 converts into a fourth form of triphosphorus pentanitride, α′‑P3N5.

The structure of all polymorphs of triphosphorus pentanitride was determined by single crystal X-ray diffraction. α‑P3N5 and α′‑P3N5 are formed of a network structure of PN4 tetrahedra with 2- and 3-coordinated nitrides, γ‑P3N5 is composed of both PN4 and PN5 polyhedra while δ-P3N5 is composed exclusively of corner- and edge-sharing PN6 octahedra. δ-P3N5 is the most incompressible triphosphorus pentanitride, having a bulk modulus of 313 GPa.

Potential applications
Triphosphorus pentanitride has no commercial applications, although it found use as a gettering material for incandescent lamps, replacing various mixtures containing red phosphorus in the late 1960s. The lighting filaments are dipped into a suspension of P3N5 prior to being sealed into the bulb. After bulb closure, but while still on the pump, the lamps are lit, causing the P3N5 to thermally decompose into its constituent elements. Much of this is removed by the pump but enough P4 vapor remains to react with any residual oxygen inside the bulb. Once the vapor pressure of P4 is low enough, either filler gas is admitted to the bulb prior to sealing off or, if a vacuum atmosphere is desired, the bulb is sealed off at that point. The high decomposition temperature of P3N5 allows sealing machines to run faster and hotter than was possible using red phosphorus.

Related halogen containing cyclic polymers, trimeric hexabromophosphazene (PNBr2)3 (melting point 192 °C) and tetrameric octabromophosphazene (PNBr2)4 (melting point 202 °C) find similar lamp gettering applications for tungsten halogen lamps, where they perform the dual processies of gettering and precise halogen dosing.

Triphosphorus pentanitride has also been investigated as a semiconductor for applications in microelectronics, particularly as a gate insulator in metal-insulator-semiconductor devices.

As a fuel in pyrotechnic obscurant mixtures, it offers some benefits over the more commonly used red phosphorus, owing mainly to its higher chemical stability. Unlike red phosphorus, P3N5 can be safely mixed with strong oxidizers, even potassium chlorate. While these mixtures can burn up to 200 times faster than state-of-the-art red phosphorus mixtures, they are far less sensitive to shock and friction. Additionally, P3N5 is much more resistant to hydrolysis than red phosphorus, giving pyrotechnic mixtures based on it greater stability under long-term storage.

Patents have been filed for the use of triphosphorus pentanitride in fire fighting measures.